It is known to disable a vehicle occupant protection device, such as an air bag for an occupant of a vehicle seat, if it is determined that an occupant of the seat is under a certain weight. There are various known seat weight sensing systems that determine the weight or load on the vehicle seat. These known seat weight sensing systems typically include weight sensors that are mounted on the vehicle seat. It is also known to distinguish between the weight of an adult seated in the seat, of a child seated in the seat, and of a child seat cinched down tight in the vehicle seat and, in response, to permit, inhibit, or tailor actuation of the protection device.
Seat weight sensing systems may include strain gauges fixed to a vehicle seat frame member in order to sense strain in the member caused by changing loads on the vehicle seat. These strain gauges may be arranged in a bridge circuit in, order to eliminate temperature errors. The bridge circuit functions to eliminate these errors when all of the strain gauges in the circuit are at the same temperature.
During periods where the vehicle HVAC system is used to warm-up or cool down the vehicle, some strain gauges in the bridge circuit may warm-up or cool down more rapidly than others. As a result, temperature differentials between the individual strain gauges in the bridge circuit may occur and result in erroneous variations or “drift” in the sensed load. Drift may also occur as the result of fatigue in the parts upon which the sensor is mounted and as the result of aging of the sensor or sensor components.
Since the strain gauges are typically at the same ambient temperature when the vehicle initially loaded, the initial sensed load can typically be accurate. As the vehicle is heated or cooled from ambient temperature conditions, temperature transients between the strain gauges in the bridge circuit may occur. The duration of temperature transients can vary depending on factors, such as the ambient temperature from which the vehicle is heated or cooled by the HVAC system. As an example, the heating or cooling period may last for 15-20 minutes after activating the HVAC system. Once the heating or cooling period has lapsed and the strain gauges in the bridge circuit are operating at the same temperature, the temperature transients no longer pose a problem.